Re: [PATCH] zram: add restriction on dynamic zram device creation

[Minchan Kim <minchan@kernel.org>]

Hello, Yi,

On Tue, Sep 01, 2020 at 02:42:47PM +0800, Yi Wang wrote:
> From: zhanglin <zhang.lin16@zte.com.cn>
> 
> Add max_num_devices to limit dynamic zram device creation to prevent
>  potential OOM
> 
> Signed-off-by: zhanglin <zhang.lin16@zte.com.cn>
> ---
>  drivers/block/zram/zram_drv.c | 26 +++++++++++++++++++-------
>  1 file changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 36d49159140f..4f2c4eef5051 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -43,8 +43,9 @@ static DEFINE_MUTEX(zram_index_mutex);
>  static int zram_major;
>  static const char *default_compressor = "lzo-rle";
>  
> -/* Module params (documentation at end) */
>  static unsigned int num_devices = 1;
> +/* Module params (documentation at end) */
> +static unsigned int max_num_devices = 8;

It seems 8 is too small to me by default. How about 256?
Furthemore, I think it would be worth to have in Kconfig.

config ZRAM_DEV_MAX_COUNT
	int "Number of zram devices to be created"
	depends on ZRAM
	default 256
	help
	   blah blah

>  /*
>   * Pages that compress to sizes equals or greater than this are stored
>   * uncompressed in memory.
> @@ -2013,10 +2014,16 @@ static ssize_t hot_add_show(struct class *class,
>  			struct class_attribute *attr,
>  			char *buf)
>  {
> -	int ret;
> +	int ret = -ENOSPC;
>  
>  	mutex_lock(&zram_index_mutex);
> +	if (num_devices >= max_num_devices) {
> +		mutex_unlock(&zram_index_mutex);
> +		return ret;
> +	}
>  	ret = zram_add();
> +	if (ret >= 0)
> +		num_devices += 1;
>  	mutex_unlock(&zram_index_mutex);
>  
>  	if (ret < 0)
> @@ -2046,8 +2053,10 @@ static ssize_t hot_remove_store(struct class *class,
>  	zram = idr_find(&zram_index_idr, dev_id);
>  	if (zram) {
>  		ret = zram_remove(zram);
> -		if (!ret)
> +		if (!ret) {
>  			idr_remove(&zram_index_idr, dev_id);
> +			num_devices -= 1;
> +		}
>  	} else {
>  		ret = -ENODEV;
>  	}
> @@ -2089,6 +2098,7 @@ static void destroy_devices(void)
>  static int __init zram_init(void)
>  {
>  	int ret;
> +	unsigned int i;
>  
>  	ret = cpuhp_setup_state_multi(CPUHP_ZCOMP_PREPARE, "block/zram:prepare",
>  				      zcomp_cpu_up_prepare, zcomp_cpu_dead);
> @@ -2111,13 +2121,15 @@ static int __init zram_init(void)
>  		return -EBUSY;
>  	}
>  
> -	while (num_devices != 0) {
> +	if (num_devices > max_num_devices)
> +		num_devices = max_num_devices;

I am not sure the overriding is a good idea.
Just leave it but just return error if user wanted to pre-create
exceeding max size in configuration.

Thanks.

> +
> +	for (i = 0; i < num_devices; i++) {
>  		mutex_lock(&zram_index_mutex);
>  		ret = zram_add();
>  		mutex_unlock(&zram_index_mutex);
>  		if (ret < 0)
>  			goto out_error;
> -		num_devices--;
>  	}
>  
>  	return 0;
> @@ -2135,8 +2147,8 @@ static void __exit zram_exit(void)
>  module_init(zram_init);
>  module_exit(zram_exit);
>  
> -module_param(num_devices, uint, 0);
> -MODULE_PARM_DESC(num_devices, "Number of pre-created zram devices");
> +module_param(max_num_devices, uint, 0);
> +MODULE_PARM_DESC(max_num_devices, "Max number of created zram devices");
>  
>  MODULE_LICENSE("Dual BSD/GPL");
>  MODULE_AUTHOR("Nitin Gupta <ngupta@vflare.org>");
> -- 
> 2.17.1
>